Transcript More Genetics

More Genetics
Complete Dominance
• The pattern we’ve seen so far
• Homozygous dominant and
heterozygous are indistinguishable
• YY and Yy will both make yellow seed
Complete Dominance
• Autosomal recessive pattern::
• AA, Aa are “normal”
• aa has the condition
• Autosomal dominant pattern:
• AA, Aa have the condition
• aa is “normal”
Some patterns of inheritance don’t follow the rules.
• Incomplete dominance
• Heterozygotes have a blended appearance
between both alleles
• Ex. Blended colors in flowers
• Codominance
• Heterozygotes have the phenotype of both
alleles expressed at the same time
• Ex. Roan horses (next slide)
What would you expect in each situation?
INCOMPLETE DOMINANCE
• Red flower x white flower =
pink
CODOMINANCE
• Red flower x white flower =
red and white flower mix
Multiple Alleles
• Instead of just two alleles
(A vs. a), more than two alleles
control a trait
• Human blood types work this
way
• Alleles = IA, IB, i
• IA & IB = Codominant
Blood Typing
Phenotype
(Blood Group)
Genotype(s)
Type A
IAIA or IAi
Type B
IBIB or IBi
Type AB
IAIB
Type O
ii
Blood Typing Problem:
• A man who is heterozygous for Type A blood marries a woman who is
homozygous for Type B blood. What possible blood types might their
children have?
Blood Typing Problem 2:
• A woman is IAIB. She has a child that is Type AB. Which genotypes might the
father have had?
Polygenic Inheritance: the
effect of 2 or more genes acting
upon a single phenotypic
character (eg. skin color, height)
These girls are fraternal twins!
• http://www.parentdish.co.uk/2012/03/31/beautiful-black-and-white-twins-kian-andremee-hodgson-turn-seven/
Pleiotropy: one gene influences multiple, seemingly unrelated phenotypic traits
Note the differences!
• Multiple alleles
• More than two
alleles code for
a trait
• IA, IB, i
• Polygenic
inheritance
• More than two
genes influence a
trait
• Gene 1, gene 2,
gene 3, gene 4 all
influence skin color
• Pleiotropy
• One gene influences
many phenotypes
• The “Frizzle Gene”
mutation makes
chickens have curled
feathers. This ALSO
causes issues with
body temperature,
metabolism, blood
flow, digestion.
Lethal Alleles
• First, cross Ll x Ll
• Some genotypes are lethal and cause death (generally, before an
organism is born)
• LEAVE OUT LETHALS when figuring genotype, phenotype, and
probability. DEAD IS DEAD! You have no phenotype if you are dead.
• The ratios that would result if ll was lethal:
Practice
• For a certain disease, HH is a lethal allele that causes death before an
organism is born. If two parents, Hh x Hh, have kids, what is the
chance that they will have a child that is homozygous recessive?
Some traits are non-nuclear
• Genes on chloroplasts and
mitochondrial DNA do not follow
Mendelian rules
• During meiosis, chloroplasts and
mitochondria are randomly assorted to
gametes.
• Also, since mitochondria are passed via
the egg only, certain traits come only
from your mother
Nature and nurture
• Both genetic and environmental factors influence phenotype
• Example: hydrangeas change color/intensity based on the pH and
aluminum content of the soil
Pedigrees- diagrams to show the relationship
between parents/offspring across 2+ generations.
Identify each pedigree as autosomal dominant
inheritance, autosomal recessive, or neither.
Identify each pedigree as autosomal dominant
inheritance, autosomal recessive, or neither.
Identify each pedigree as autosomal dominant
inheritance, autosomal recessive, or neither.
Identify each pedigree as autosomal dominant
inheritance, autosomal recessive, or neither.
Genetic Testing
• Most children with recessive disorders are born to parents with
normal phenotypes, so determining if parents are heterozygous is
useful.
• Many tests are around that can screen for carriers of disorders like
Tay-Sachs, sickle-cell, CF, and others.
• Consider health insurance. What sorts of ethical issues might knowing your
phenotype (especially if a disease will manifest later) bring up?
• What about employers—if they don’t understand what “carrier” means and
they get a hold of your data, what might that lead to?
• Tests are out there for Huntington’s. Would you want to know?
Genetic Testing
• You will read more about genetic testing and ethical issues in your
book. 279-281
Autosomal Recessive Disorder: Cystic Fibrosis
• Most common lethal genetic disease
in the U.S.
• Normal allele: codes for membrane
protein that transport chloride ions
• Mutant allele: codes for defective (or
no) membrane protein
• Pleiotropic effects: poor absorption
of nutrients from intestines, chronic
bronchitis, recurrent bacterial
infections
Autosomal Recessive Disorder: Sickle Cell
Disease
• Most common inherited disorder
among those of African descent
• Point mutation in hemoglobin gene
• When oxygen is low, the sickle cell
hemoglobin molecules change
shape, which may clump and clog
blood vessels
Autosomal Recessive Disorder: Sickle Cell
Disease
• Phenotype varies due to genotype.
• ss = sickle cell disease = full sickle cell disease
• Ss = sickle cell trait = usually healthy, but may have some
sickle-cell symptoms during prolonged periods of reduced
blood oxygen
• SS = normal = no sickle cell at all
• IMPORTANT! Sickle cells cannot be infected by malaria.
• So…there is an advantage to being Ss…if you have sickle
cell trait, you can’t get malaria and you are also mostly
normal.
Autosomal Dominant Disorder: Huntington’s Disease
• Is a lethal dominant…but the
“lethal” part kicks in after
reproductive age
• Degenerates nervous system
• No phenotypic effects until age
35-45; irreversible and fatal
Autosomal Dominant Disorder:
Achondroplasia
• Homozygous dominant is lethal
• Heterozygous individuals will
have the dwarf phenotype
• Homozygous recessive individuals
(99.99% of the population!) are
not dwarfs
• How can a family with dwarf parents
have a non-dwarf child?